Helmontz’s Double siren (Polyphonic siren)

The dual siren is a device which produces sounds from two sources in a way that allows the phase of one sound to be varied relative to the phase of the other. It can produce any sound on a scale from semitone to an octave.
It consists of two coaxial Dove sirens (disc rotates at high speed). The air flows through a series of holes in each disk and produces a distinct sound. The frequency depends on the speed of rotation of the discs and the distance between the holes.
On top of the device there is a crank for adjusting the phase of the upper disc relative to the bottom disc. Moreover, each disc has multiple coaxial rows of holes which can be closed or open and thereby produce multiple sounds. Each disc contains 8 rows of coaxial holes. The frequency of the sound is determined by operating the siren for a known period of time and then taking the readings on the front part of the device which measure the total number of revolutions (rotations).
Hermann von Helmholtz published a description of the device in his book: «Die Lehre von den Tonempfindungen als physiologische Grundlage für die Theorie der Musik» (On the Sensations of Tone as a Physiological Basis for a Theory of Music).


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